Fix & clarify hierarchical flows

[harrisonliew]

This fixes a small bug with the hierarchical VLSI flow for power-par and adds clarifying documentation nothing that hierarchical flows must be cognizant about overriding VLSI_TOP.

@bdngo @moresa75

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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